This invention relates generally to the art of producing superhard materials and more specifically to alloys used for metallization and brazing of abrasive materials.
At the present time a great many novel artificial superhard abrasive materials are available, fabricated on the base of diamond, cubic boron nitride, etc.
Inasmuch as the newly-obtained abrasives feature properties other than those possessed by old ones, problems arise of providing novel alloy materials suitable for brazing and metallization of the abrasives, i.e., metal-facing with an alloy to reinforce abrasive grain or the product as a whole. As practical experience shows, the known alloys for brazing and metallization fail to completely meet the requirements imposed by new abrasive materials.
Thus, for instance, synthetic abrasives based on cubic boron nitride or diamond feature very low (700.degree. to 1100.degree. C) temperatures of transition to a hexagonal modification which requires low-temperature alloys for metallization and brazing; on the other hand, synthetic abrasives based on cubic boron nitride feature high chemical stability which in turn requires high adhesion on the part of alloys for brazing and metallization.
At the present time, brazing alloys for carbon-containing abrasive materials are known to be in practical use, in particular, for diamond and graphite abrasives, the alloys being based on copper, silver or gold doped with additives of iron, cobalt and nickel taken either separately or in combination with one another (see, for example, Patent No. 1,207,849 of the German Federal Republic).
Known also are brazing alloys for diamond, silicon carbide, boron carbide and corundum, such as, copper-titanium; silver-titanium, gold-titanium, tin-titanium, lead-titanium, copper-molybdenum, copper-zirconium, copper-vanadium, gold-tantalum, gold-niobium, copper-silver-titanium, copper-gold-titanium, bronze-titanium and copper-tin-titanium, the content of Ti, Mo, Zr and V in the alloys amounting to 10 weight percent. (see, for example, British Patents: No. 989,251; No. 1,100,446; No. 931,672; No. 1,013,337; No. 933,921. Patents of GFR: No. 1,210,300; No. 1,151,666; US Patents: No. 3,192,620; No. 2,570,248; French Patents: No. 1,322,423; No. 1,240,395; "Wetting and Interaction of Metal Melts with the Surface of Diamond and Graphite", Yu.V. Naidich and G. A. Kolesnichenko, "Naukova dumka" Publishers, Kiev 1967 (in Russian).
All of the brazing alloys mentioned above possess low adhesion to such abrasives as cubic boron nitride and corundum, and therefore cannot ensure proper brazing or metallization.
Known in the art are also the following brazing alloys: copper-titanium, silver-titanium and copper-silver-titanium featuring a titanium content amounting to 15 weight percent (see, for example, British Patent No. 932,729; Patent of GFR No. 1,151,666).
The brazing alloys have but a limited field of application, inasmuch as they fail to exhibit strong adhesion to all abrasives; thus, with respect to cubic boron nitride adhesion is low and insufficient to effect firm brazing and uniform coating in the process of metallization.
Another brazing alloy is known to use for diamonds, which is essentially an alloy of gold with 1 to 25 weight percent of tantalum (see, for example, U.S. Pat. No. 3,192,620). The cardinal disadvantage inherent in the alloy resides in the fact that it has a high liquid-phase point (above 1050.degree.) and therefore is restricted but to a narrow field of application, since at 1050.degree. C and over such abrasives as diamond and cubic boron nitride are liable to vigorously pass into hexagonal modification which rather adversely affects the strength of the abrasives.
One more diamond brazing alloy is now in common use, consisting of 75 weight percent of copper and 25 weight percent of titanium.
Principal disadvantages of the alloy is that it is brittle and its thermal expansion factor badly differs from that of the relevant abrasives. All of this inescapably results in thermal stresses arising in the finished products which, in turn, are liable to inflict rapid destruction thereof in the course of operation (manifesting in cracks or chippings) and, consequently, high and premature wear of the tool made of such abrasives.
Also, used for brazing diamond and graphite is silicon or aluminium (both per se) (see, for example, Patent of GFR No. 2,031,915); however, either of these has but a restricted sphere of usage, viz., silicon -- due to high melting point (1450.degree.) at which, as has been discussed above, a vigorous transition of diamond to a hexagonal modification occurs, while aluminum -- has a high oxidizability and low strength.
All of the brazing alloys described above are used also for metallization of abrasives made of diamond, cubic boron nitride, corundum, etc.
Apart from the alloys discussed above, there are also known some alloys and single metals for surface metallization of abrasives, viz., diamond, cubic boron nitride, silicon carbide and tungsten carbide, the metallization being either single- or multiple-layer. In case of multiple-layer metallization, e.g., for establishing the initial layer, use is made of nickel, copper, zinc, tin, gold, lead or their alloys; for establishing a second layer use is made of an iron-nickel alloy; and for the formation of a third layer copper or bronze is used (see, for example, Patent of GFR No. 2,021,299). Such coatings suffer from the disadvantage that owing to poor adhesion they cohere inadequately to the surface of abrasive materials and therefore are readily separated therefrom even under low applied forces. This fact seems to be explained by the weak mechanical adhesion which occurs between the coating and the base material.
As a result, the abrasive is liable to readily chip during tool operation due to rapid destruction of the coating.
In the case of two-layer metallization coatings, use is made independently of such metals as nickel, copper, cobalt, iron, chromium, as well as their alloys, the sequence of the layers and their arrangement leaving beyond preliminary specification as having no matter (see, for example, French Patent No. 2,093,564). The disadvantage featured by such coatings is their poor adhesion to the abrasive surface. In the case of a two-layer metallization for diamond, titanium is used for the initial layer, while for a second layer iron, nickel, cobalt and alloys thereof are used (see, for example, French Patent No. 2,093,865).
Used for metallization are also nickel, cobalt, silver, copper, molybdenum, titanium, aluminium, manganese, cadmium, tin, zinc, chromium, tungsten, iron, zirconium, niobium, osmium, palladium, platinum, tantalum and their alloys (see, for example, British Patents: No. 1,114,353; No. 1,154,598).
Used for single-layer metallization of abrasive materials, in particular, diamond, corundum, etc. use is made of molybdenum, titanium (as titanium hydride), zirconium (as zirconium hydride), tungsten, tantalum, as well as aluminum (see, for example, Patents of GFR: No. 2,021,399 and No. 2,010,183; British Patent No. 1,100,446; U.S. Patents: No. 2,961,750; No. 3,351,543; No. 2,570,248).
A common disadvantage of the metals or alloys is that they have but a limited field of application, since on account of their high melting points they can be used only as solid-phase coatings applied to diamond or cubic boron nitride and cannot be used as liquid brazing alloys. One more disadvantage inherent in the alloys is their low plasticity which very badly tells on their use as brazing alloys.